New Zealand and Australian authorities have helped in an international operation that brought to justice a man described as an extreme paedophile.
A university researcher described in a British court as an extreme paedophile has been jailed for 32 years for trading images of women and children on the dark web.
Twenty-nine-year-old Matthew Falder, who worked at the University of Birmingham, admitted more than 130 offences, including blackmail and rape.
He was caught as part of an international taskforce after the FBI began looking into dark-web paedophilia sites.
Scott Crabb from the US Department of Homeland Security said it was the worst case of sexual exploitation he had seen.
"It's evil. There's really no other word for it. I've just never seen anything like this where someone is willing to go to these extremes to torment people. There is a message that you can operate for a while on the internet and get away with it but at some point law enforcement is going to catch you."
Investigators enlisted the help of authorities in New Zealand, Australia, Israel and other countries to catch Falder, who was a graduate of Cambridge and a researcher in geophysics at the University of Birmingham.
A court was told the 29-year-old used the dark web to blackmail victims and encouraged the rape of a child, pretending to be a female artist and tricking victims into sending him humiliating images of themselves.
Falder was part of a subculture of paedophilia called "hurtcore" and police said he was the worst sex offender they had encountered operating on the internet.
He admitted to 137 offences, including blackmail, voyeurism and encouraging the rape of a child.
Police video of Falder's arrest shows him, apparently handcuffed and wearing a T-shirt, asking officers "what is it that I'm supposed to have done?".
"That sounds like the rap sheet from hell," he said after hearing the list of charges against him.
Posing as an artist looking to do life drawings, Falder lured victims into sending him images, many of which ended up on the dark web. He approached 300 people worldwide, some of them teens advertising for babysitting or dog walking jobs online.
Judge Philip Parker branded Falder an "internet highwayman", whose behaviour was "cunning, persistent, manipulative and cruel".
"The damage is ongoing for these individuals," Judge Parker said of Falder's victims.
Britain's National Crime Agency (NCA) said Falder's crimes required unprecedented levels of resources to stop.
The agency worked with the country's electronic intelligence agency, US Homeland Security, the AFP and Europol to crack the case. At one point, some 100 investigators were involved.
"In more than 30 years of law enforcement, I've never come across an offender whose sole motivation was to inflict such profound anguish and pain," said Matt Sutton, a NCA senior investigating officer.
"I've also never known such an extremely complex investigation with an offender who was technologically savvy and able to stay hidden in the darkest recesses of the dark web.
"This investigation represents a watershed moment."
Falder had been working as a lecturer at the University of Birmingham when arrested by the NCA last year.
Adept at covering his tracks, he used 70 online identities - careful not to leave tracks on the social media.
He had an account on the Hurt 2 The Core network, an encrypted site on the dark web taken down by the FBI, which alerted British police.
The NCA, the British equivalent of the FBI, had little to go on save the online alias "inthegarden".
"I had no scene, the internet is a virtual scene. I had no forensics whatsoever, nothing, no trace whatsoever and no witnesses …" Mr Sutton said.
There was no money trail to follow, either, as Falder sought status among others with similar proclivities, not cash.
"He was not about money, his currency was his kudos in the community, his standing in this world, and he traded in these type of images and this type of control," Mr Sutton said.
Authorities contrasted Falder's level of offending against his academic prowess. A graduate of Cambridge University, he led a double life and was in a relationship.
Ruona Iguyovwe of the Crown Prosecution Service said he was very "IT savvy".
"During the day he's a lecturer in geophysics at Birmingham University, while at night online on his computer in the privacy of his iPad or his encrypted email address, he was 'evilmind' or '666devil,'" she said.
- ABC/ BBC